1. Field of the Invention
The present invention relates to a recording medium as well as a method for reading data from and writing data on the medium. A magnetic or magneto-optical recording medium has a large storing capacity, a high degree of reliability and a good portability as a removable recording medium. Therefore, the application field of the magneto-optical recording medium is rapidly widened for a data recording medium of a computer or a recording medium of image information. As the wide application, the request of the market for higher density and larger capacity has been increasing every year.
2. Description of the Prior Art
There are two methods for increasing a recording density (i.e., increasing a recording capacity) of a disk recording medium in which data are written linearly along tracks. The first method increases a bit density on a track, while the second method increases a track density.
The first method has a limitation caused by a diameter of the light beam spot for a magneto-optical recording medium. In order to read a bit written in a period smaller than a beam spot diameter, a smaller diameter of the light beam spot is required. However, the beam spot diameter cannot be smaller than the value determined by a wavelength xcex of a light source and a numerical aperture NA of an object lens.
Recently, a technique of reading a bit recorded in a period smaller than the beam spot diameter is proposed in Japanese unexamined patent publication No. 1-143041 or No. 3-93058. This technique utilizes a temperature distribution within the beam spot on the medium and making the recording medium in a multilayer structure for obtaining the effect as if the beam spot is focused more precisely.
FIG. 1 shows schematically the structure of the conventional recording medium and a method for recording and reproducing data using the medium disclosed in the Japanese unexamined patent publication No. 1-143041. This recording medium has three layers, i.e., a reproduction layer 101, an intermediate layer 102 and a recording layer 103. The bit 105 recorded in the recording layer 103 by a light modulation method is transferred from a low temperature area 104a in a beam spot 104 to the reproduction layer 101 by using an exchange connection via the intermediate layer 102 so as to reproduce the bit 105. In a high temperature area 102a of the intermediate layer 102 whose temperature exceeds the Curie temperature, the exchange connection is interrupted and the magnetization of the reproduction layer 101 is directed to the magnetic field Hr for reading applied externally. Namely, a bit 105a of the recording layer 103 is hidden (is masked magnetically). Thus, among bits located in the beam spot 104, only a bit located in the low temperature area 104a can be read. Accordingly, the reproducing resolution can be improved as if the beam spot is focused more precisely.
FIG. 2 shows schematically the structure of the recording medium and a method for reading data from and writing data on the medium disclosed in the Japanese unexamined patent publication No. 3-93058. This recording medium has four layers, i.e., a reproduction layer 201, a control layer 202, an intermediate layer 203 and a recording layer 204. In this recording medium, the magnetization of the reproduction layer 201 forms the state where bits are hidden in the low temperature area 205a and the high temperature area 205b within the beam spot 205. Only in a medium temperature area 205c, the bit is transferred from the recording layer 204 to the reproduction layer 201 to be read by the exchange connection. The magnetized state of the reproduction layer 201 in the high temperature area 205b is directed to the magnetic field (reproduction magnetic field) Hr applied externally by the same principle as the recording medium explained above.
In the low temperature area 205a, the magnetized state of the reproduction layer 201 keeps the magnetization direction formed by an initialization magnetic field HO (usually, a magnetic field of a few kilo Oe). This is caused by the fact that the composed coercive force of the reproduction layer 201 and the control layer (also referred to as an auxiliary reproduction layer) 202 in the low temperature area 205a is larger than the exchange connection force from the recording layer 204 via intermediate layer 203. The medium temperature area 205c, i.e., a transfer area is formed when the above-mentioned inequality is inverted as the temperature rises.
The above-mentioned method for reading data from the magneto-optical recording medium having three or four layer structure is called a magnetic super resolution (MSR) method. FIGS. 3A and 3B show the relationship among the applied magnetic field, the laser light emission, the recorded mark and the reproduced data when using the conventional recording medium shown in FIG. 1 or FIG. 2. FIG. 3A shows the case of magnetic field modulation method in which the laser emits constant light, while FIG. 3B shows the case of magnetic field modulation method in which the laser emits pulsed light. FIGS. 3A and 3B are diagrams explaining the magnetic field modulation recording method. There is another method of light modulation recording method. FIGS. 3A and 3B show an example (the magnetic field modulation) of the recording methods, and another method (light modulation) can be used.
FIG. 4 is a block diagram showing the structure of the conventional recording and reproducing apparatus using the magneto-optical recording medium. A magnetic coil drive circuit 304 drives a magnetic coil 303 so as to generate the magnetic field for reading. The read signal obtained by the optical head 302 is amplified by an amplifier 305, is adjusted about its gain by an automatic gain control (AGC) amplifier 306, is equalized about its waveform by an equalizer 307, and the high frequency noise of the signal is removed by a low pass filter (LPF) 308. The signal is further digitized into a binary signal by a binary circuit 309 and becomes separate data after passing through a data discriminator 311 and a phase-locked loop (PLL) circuit 312. The separate data are given to a demodulator 313, which demodulates the data.
FIG. 5 shows a dependency of the repeatedly recorded mark on the magnetic field for reading. Namely, FIG. 5 shows a relationship between the applied magnetic field for reading that is necessary when reading data from the recording medium and a ratio of carrier and noise (CNR). It is understood from FIG. 5 that the CNR becomes larger than a predetermined level (CNRO) when the applied magnetic field for reading is within a predetermined range (the optimal reading magnetic field). If the applied magnetic field for reading is too stronger or too weaker than the optimal reading magnetic field, the signal will be deteriorated.
FIG. 6 shows data reproduction waveform and the binary data signal when the applied magnetic field for reading is a negative magnetic field. The reproduced waveform obtained by applying the negative magnetic field has a milder gradient in the leading edge than in the trailing edge, so a jitter of the binary signal for the same noise power becomes larger in the leading edge than in the trailing edge. Therefore, the quality of the reproduced data depends on the jitter in the leading edge.
FIG. 7 shows data reproduction waveform and the binary data signal when the applied magnetic field for reading is a positive magnetic field. In this case, on the contrary to the case of the negative magnetic field, the reproduced waveform has a milder gradient in the trailing edge than in the leading edge, so a jitter in the trailing edge determines the quality of the reproduced data.
As explained above, when the magnetic super resolution method used for reading a small bit recorded on a three layer or four layer structure of recording medium, the read signal becomes asymmetric so that the gradient of the leading edge or the trailing edge becomes mild, resulting in a deterioration of a quality of reproduced data. In addition, the second method for improving the recording density includes the method of narrowing the track width so as to improve the track density and the method of recording data on both lands and grooves. When improving the track density by these methods, a cross talk problem will be caused by a mark of the neighboring track that enters the beam spot for reading the present mark. In order to solve this problem, a method disclosed in Japanese unexamined patent publication No. 1-143041 or No. 3-93058 can be used. This method can read only the mark of the current track to be read by virtually decreasing the effective beam spot diameter. Furthermore, as shown in FIG. 8, there is a method for suppressing the influence of the cross talk by arranging the recorded marks in a staggered configuration between the neighboring tracks (see Japanese unexamined patent publication No. 6-76399, for example).
In order to achieve higher density of a disk recording medium, it is necessary to use a technique to improve a bit density and a technique to improve a track density simultaneously. In this case, since the mark is recorded by magnetization recording, there is a problem that the leakage magnetic field from the neighboring recorded mark affects the current track. This is another problem different from the above-mentioned cross talk in the narrow meaning.
The ill effect of this leakage magnetic field cannot be improved even if the effective beam spot diameter is decreased virtually as described in Japanese unexamined patent publication No. 1-143041 or No. 3-93058. Namely, the ill effect of the magnetic field cannot be suppressed. In addition, the width of the recorded mark in the magnetic field modulation recording method that enables the higher density recording is generally wider than that in the light modulation recording method, so it is affected by the leakage magnetic field of the neighboring track more easily.
Particularly, when using the magnetic field for reading, the effective magnetic field that the recorded bit of the current track receives from the leakage magnetic field of the neighboring track changes and becomes out of the range of the optimal reading magnetic field shown in FIG. 5. As a result, the recorded data of the current track cannot be read correctly due to the ill effect of the leakage magnetic field of the neighboring track.
In addition, if an even track is to be read, the recording method shown in FIG. 8 increases the influences of the leakage magnetic fields of the neighboring tracks (odd tracks), since they have the same polarity. Namely, noting the leading edge (the left side) of the recorded bit of the even track in the center as shown in FIG. 8, the both neighboring odd tracks are magnetized in the positive polarity. Noting the trailing edge, the both neighboring odd tracks are magnetized in the negative polarity. In this way, as the ill effect of the leakage magnetic field from the tracks neighboring the current track increases, the effective magnetic field that the recorded bit of the current track receives go out of the range of the optimal reading magnetic field, so that the correct reproduction cannot be performed.
Furthermore, as explained above, the waveform of the read signal becomes asymmetric under a constant magnetic field applied for reading, and the gradient of the leading edge or the trailing edge becomes mild. As a result, the jitter at the edge with the mild gradient is deteriorated, so that the quality of the reproduced data is deteriorated.
The object of the present invention is to provide a magneto-optical recording medium, a method for writing data on and reading data from the medium and a magneto-optical recording apparatus, in which the recorded signal is read by an optical head while applying a magnetic field, and an ill effect of the leakage magnetic field from the neighboring track can be suppressed so as to obtain a correctly read signal.
A magneto-optical recording medium according to the present invention uses a light beam irradiated externally and a magnetic field applied externally for reading the recorded data. Each of leading and trailing edges of a recorded mark is substantially aligned with center portions of a recorded mark or a space of neighboring tracks, and the neighboring tracks have different polarities.
A first method for writing data on and reading data from a magneto-optical recording medium according to the present invention uses a light beam irradiated externally and a magnetic field applied externally. The method comprises the steps of writing marks so that each of leading and trailing edges of a recorded mark is substantially aligned with center portions of a recorded mark or a space of neighboring tracks, and that the neighboring tracks have different polarities; and reading a signal including the recorded mark in the condition where influences of leakage magnetic fields of the neighboring tracks counterbalance each other by positioning a light beam spot for reading at substantially the center in the width direction of the track to be read.
According to the above-mentioned magneto-optical recording medium and the method, the influences of the leakage magnetic fields from the tracks neighboring the current track to be read are canceled, so that the recorded signal of the current track is correctly read with being hardly affected by the leakage magnetic field from the neighboring tracks.
A second method for writing data on and reading data from a magneto-optical recording medium according to the present invention comprises the steps of writing marks so that each of leading and trailing edges of a recorded mark is substantially aligned with center portions of a recorded mark or a space of neighboring tracks, and that the neighboring tracks have different polarities; and reading a signal including the recorded mark in the condition where the external magnetic field plus a leakage magnetic field of one of the neighboring tracks is the optimal reading magnetic field by shifting a light beam spot for reading in the width direction of the track from the center to one of the neighboring tracks and by setting the intensity of the external magnetic field weaker than the optimal reading magnetic field.
According to the second method, the leakage magnetic field from the neighboring track is utilized for reducing the necessary magnetic field to be applied for reading, so that the magnetic coil can be downsized and the power for driving the coil can be reduced. In addition, as the magnetic field is reduced, the magnetic field can be switched more rapidly for a high speed transmission or a high density recording. Since the leakage magnetic fields of the tracks neighboring the current track to be read have opposite direction to each other in the magneto-optical recording medium of the present invention, one of the leakage magnetic fields undoubtedly has the same direction as the magnetic field for reading, i.e., the direction that strengthens the magnetic field for reading.
Preferably, the first or the second method explained above further includes the step of switching the polarity of the external magnetic field for the leading edge and the trailing edge of the recorded mark. Thus, a sharp waveform of read signal can be obtained in both the leading edge and the trailing edge of the recorded mark, so that a jitter is reduced. Accordingly, any one of the edges or the both edges can be used for reproducing the recorded data.
It is also preferable that the writing step of the first or the second method includes the steps of shifting the position of at least one of the leading and trailing edges of the recorded mark along the track for multilevel recording in which a recorded mark has plural bit information. Especially, the sharp edge of the reproduced waveform facilitates the multilevel recording by shifting the position of the edge.
A magneto-optical recording apparatus according to the present invention comprises a magneto-optical recording medium from which recorded data are read by using a light beam irradiated externally and a magnetic field applied externally; means for applying the magnetic field for reading to the magneto-optical recording medium; an optical head for irradiating a light beam onto the magneto-optical recording medium and for reading a signal recorded in the magneto-optical recording medium in accordance with the light after reflected by the magneto-optical recording medium; and means for determining the optimal value or the optimal range of the magnetic field to be applied externally in accordance with a signal read by the optical head from a reference area of the magneto-optical recording medium. This magneto-optical recording apparatus can read the recorded signal correctly since the optimal value or the optimal range of the magnetic field to be applied externally is automatically determined. In addition, this apparatus can easily realize the second method explained above.